1. Field of the Invention
This invention relates to frame-rate converting film scanners of the type having a line sensor for scanning an image of the film in one direction and a rotating multifacet mirror for displacing the image relative to the line sensor in a direction generally perpendicular to the line scan direction, and more particularly to a control system for controlling the speed of rotation of the rotating mirror relative to the speed of the film drive in such a scanner.
2. Discussion Related to the Problem
Apparatus for scanning motion picture film to produce a television signal is well known. In such film scanning apparatus, a film frame is scanned in a direction generally perpendicular to the length of the film at a standard television line rate. The line scan is displaced in a direction generally parallel with the length of the film at a standard television field rate. In the past, the line scan has been accomplished by flying spot scanners employing Nipkow discs, movable reflecting elements or cathode ray tubes. The use of a solid-state line sensing array as an alternative to a flying spot scanner to provide line scanning in a film scanner has been proposed. See D. T. Wright, "Solid-State Sensors; The Use of a Single Dimension 512-Element Array for Film scanning", BBC Research Department Report No. 1973/32. The potential advantages of a line sensing array over a flying spot scanner are numerous, including: reduced size, weight, maintenance and cost.
These advantages make the solid-state line sensing array particularly attractive for use in a film scanner for displaying amateur movie film on a home television set. A technique for converting an 18 frame per second motion picture film to a 60 field per second television signal is disclosed in copending U.S. Patent application Ser. No. 68,031 entitled FILM SCANNING METHOD AND APPARATUS EMPLOYING OPTICAL SPLICE TECHNIQUE FOR FRAME-RATE CONVERSION, filed by D. G. Howe on the same day as this application. The scanning apparatus includes a solid-state line sensing array for scanning a projected image of the film in a direction generally perpendicular to the length of the film, and a rotating multifacet mirror comprising a reflecting polygon for displacing images reflected from the facets of the polygon relative to the line sensor in a direction generally parallel to the length of the film. The polygon is rotated at a rate such that the sum of the motion of the film through the projector plus the effect of the rotating polygon causes a full frame to sweep past the line sensor in the time required to scan a television field. Thus, if the film is projected at X frames per second (e.g. 18) and the polygon is rotated at Y facets per second (e.g. 42), the output signal of the scanner will represent Z fields per second, where Z=X+Y (60 fields per second in the example.) In the apparatus disclosed by Howe, the film drive and polygon drive motors are both controlled by phase locked loop servo systems to insure that the film rate plus the polygon facet rate yield the desired frame-rate conversion.
As it turns out, the horizontal synchronization signal in an NTSC television is divisible both by 18 and 42, therefore the reference signals required for the phase locked loops can be generated simply by dividing the 15,750 Hz horizontal synchronization signal by 875 to yield the 18 Hz reference; and by 375 to yield the 42 Hz reference signal. Although several other film-speed/facet-rate combinations can be achieved by employing this scheme, continuously variable film projection rates are not practically implemented. It would be desirable to provide continuously variable film projection rates in such a film scanner. The problem therefore is to provide a control system whereby, when the frame projection rate X is varied continuously over a range, the polygon facet rate Y will be automatically chosen such at X+Y=Z, where Z is the standard video field rate.